1. Field of the Invention
The present invention relates to the mixing of liquids, particularly viscous liquids, for example plastic materials such as polymers, and especially the mixing of such materials having widely different viscosities, and when a minor phase is highly viscous. However, the invention can also be used for mixing other liquids, for example milk homogenization and preparation of mayonnaise in the food industry, preparation of explosive emulsions in the explosive industry, and homogenisation of molten soaps in the chemical industry.
2. Prior Art
One form of the present invention is an improvement of the motionless extensional flow mixer described in our U.S. Pat. No. 5,451,106, issued Sep. 19, 1995, which gives a detailed review of the prior art in this field.
Briefly, it is known to mix polymers by distributive mixing effected by so-called xe2x80x9cmotionless mixersxe2x80x9d placed between a screw feeder and a die. In most cases these mixers have a number of alternating right and left-handed helical elements placed in a tubular housing equipped with temperature control. The energy for mixing is provided by the pressure loss across the mixer. The splitting and recombination of streams results in a predictable number of striations. The advantage of such mixers is that they are accessories to standard type of compounding or processing equipment, not their integral part, and their main disadvantages are lack of easy adjustment, limited effectiveness in mixing, and inability to provide dispersive mixing. The basic principle behind their design is division and recombination of the flow streams. Since the flow division is of the shear type, the dispersive forces are usually weak, limited to the cases where the two liquids show similar viscosity.
Theoretical calculations and experiments have shown that dispersive mixing of two Newtonian liquids is more efficient in extensional than in shear flow. Extensional flow occurs for example when fluid converges from a reservoir to a capillary. In shear flow fields it is impossible to disperse liquids that have viscosity higher than that of the matrix fluid by more than a factor of 3.8. By contrast, the dispersing capability of the extensional flow field is only slightly affected by the viscosity ratio. From the kinematics point of view, the extensional flow field engenders deformation much more rapidly (note the absence of vorticity in the elongational flow field). At a given stress level, the generated interphase (that is the accepted measure of adequacy of mixing or xe2x80x9cmixednessxe2x80x9d) is orders of magnitude greater than that generated in shear. Similarly, the amount of energy required to generate a given degree of mixedness in elongation is orders of magnitude smaller than that in shear. Furthermore, the mechano-chemical degradation of the macromolecules is much less extensive in the elongational than in the shear field.
In spite of all these advantages present mixing equipment (including the twin-screw extruders) operates mainly in shear. This is due to the ease of designing equipment that operates on the shear flow principle. By contrast, it is difficult to envisage geometry that will engender very large deformations in the extensional flow field. However, one may by-pass this problem by designing a mixer in which the elongational flow field is engendered in a series of convergent-divergent geometries, preferably with semi-quiescent zones in between.
One prior patent describing an extensional flow mixer was U.S. Pat. No. 4,334,783 of Suzaka, which issued Jun. 15, 1982. The drawbacks of the Suzaka mixer are described in our aforesaid ""106 patent. The mixer described in our ""106 patent was intended to overcome these drawbacks, and to provide a mixer having the following characteristics:
1. The mixture of two fluids is exposed to strong extensional flow fields, each followed by a semi-quiescent zone;
2. The flow fields are generated by a series of convergences and divergences of progressively increasing intensity;
3. To reduce the pressure drop, as well as to prevent blockage of the restrictive openings, a series of holes (e.g. of the Suzaka design) are replaced by slits;
4. The slit gaps are made adjustable.
The mixer of our ""106 patent has a series of chambers separated by several convergent/divergent surfaces providing narrow openings between the chambers. The openings are in the form of slits defined by the inner edges of protrusions formed on die members which provide the convergent/divergent surfaces. Also, the die members subject the liquids to gradually increasing stress, since the protrusions of the die members are concentric and are arranged so that during mixing the liquids pass radially inwards between the die members in passing from the inlet to the outlet of the mixer. At least one of the die members is made movable to adjust the slit gap, thereby adjusting the stress level.
In the design shown in our ""106 patent, the movable die member is held at the lower end of a cylindrical block or mandrel which is slidable in a cylindrical chamber of a housing. Movement of the block, for adjustment of the gap width, is effected by rotating a wedge-shaped disc between an end of the housing and a sloping top end of the block. Passageways for the supply of the mixed liquids to the edges of the die members are formed around the sides of the block, and communicate with a side inlet into the housing. This construction has been found to have two drawbacks.
Firstly, when using high pressures in the mixer, for example 3,000 psi or 20 MPa, the liquid pressure at the side of the block adjacent the side inlet tends to tilt the block causing asymmetrical flow to the edges of the die members. Secondly, the wedge-shaped disc used to vary the slit gaps was difficult to adjust. The present invention overcomes these problems.
Another form of the present invention combines features of the ""106 motionless mixer patent with some features of known dispersive mixers that are used in association with screw extruders, particularly single screw extruders, to improve the mixing capability of such extruders. Such mixers generally have a housing defining a cylindrical cavity with inlet and outlet ends, and a mandrel of generally cylindrical form which is rotatable in the cavity. The mandrel has protrusions which may resemble screw threads, but which are interrupted by gaps, or separated by other, discrete protrusions or indentations, so that the material being mixed is not merely progressed along the cavity, as in a screw extruder, but is also caused to move through slits between the outer edges of the protrusions and the inside surface of the cavity. The side surfaces of the protuberances provide convergent entrances into, and divergent exits from, the slits.
The extensional flow mixer of this invention is similar to that of our ""106 patent in having:
a housing providing a cavity having an internal surface, and having an inlet into the cavity which inlet is connectable to a pressurized source of the liquids, the end of the housing remote from the inlet having an outlet for the mixed liquids;
a mandrel located in the cavity;
the mandrel carrying protrusions having side surfaces which converge towards their outer edges, the outer edges cooperating with the internal surface of the cavity to divide the space between the protrusions and the internal surface into a series of chambers separated by slits such that liquid passes successively through all the chambers and slits in moving from the inlet to the outlet, the side surfaces providing convergent entrances to, and divergent exits from, the slits, and the slits having cross-sectional areas which decrease in the liquid flow direction, from an upstream chamber adjacent the inlet, to the outlet; and means for adjusting the slit gaps.
To overcome problems with asymmetrical flow of liquids into the outermost cavity, in accordance with this invention the inlet into the housing is at an end of the housing, rather than at the side, and the mandrel has a side portion provided with helical grooves which cooperate with an interior surface of the housing to form helical passageways connecting the inlet to the upstream chamber for distributing the liquids to this chamber.
In a preferred embodiment, at least one helical groove is provided for each 25 mm of the mandrel diameter, each groove leading from an inlet end of the block mandrel to the vicinity of the upstream chamber.
In the mixer of our ""106 patent, adjustment of the slit gaps was achieved by moving the block or mandrel, which carried one series of the protrusions. In accordance with another aspect of the present invention, the block or mandrel is stationary relative to a fixed part of the housing, and this fixed part of the housing is connected by screw threads to a relatively adjustable part of the housing. The relatively adjustable part may carry protuberances which cooperate with those of the mandrel to define the slits.
As indicated, one form of the present invention has features in common with known dispersive mixers having a housing defining a cavity which is usually of generally cylindrical form, and having a mandrel rotatable in the cavity, the mandrel having protrusions. However, the present invention differs from this prior art, firstly, in that the cavity is frusto-conical having a large end at the inlet and a small end at the outlet, and in that the protrusions are annular, and are such that said outer edges divide the space between the protrusions and the internal surface into a series of annular chambers separated by the slits such that liquid passes successively through all the chambers and slits in moving from the inlet to the outlet. The chambers have a mean diameter which decreases from an outermost chamber adjacent the inlet to an innermost chamber adjacent the outlet.
This form of the invention may also include screw thread means for adjusting the axial position of a portion of the housing relative to the mandrel to alter the slit gaps. Also again, the mandrel may have, adjacent the inlet, a side portion provided with helical grooves, the grooves forming helical passageways with an interior surface portion of the housing, the passageways communicating with the upstream chamber. This dynamic form of the invention, termed a dynamic extensional flow mixer (DEFM), has all four elements which constitute the fundamental principles of the invention: strong, elongational flow fields, increasing in intensity in the downstream direction, and the use of slits which are adjustable.
In this form of the invention, the fact that the mandrel rotates adds angular shear to the mixing; this is desirable as it prevents an elongated droplet from returning to a spherical shape.